1. Field of the Invention
The present invention relates to an optical scanner in which a light beam deflected by a deflecting means having a deflecting reflecting face rotated at an equal speed is converged as a light spot on a scanned face by the image forming action of a reflective image-forming element to substantially perform a scanning operation at an equal speed. The present invention can be utilized in a writing optical system in a digital copying machine, an optical printer, etc. and a scanning optical system in a measuring device, a display, etc.
2. Description of the Related Art
In an optical scanner, an f.theta. lens is generally known widely as an optical system for converging a deflected light beam as a light spot on a scanned face. Recently, a reflective image-forming element has been used instead of the f.theta. lens. This reflective image-forming element is an optical element having a function for reflecting and converging the deflected light beam as a light spot on the scanned face. In this patent specification, this optical element is called the reflective image-forming element. For example, the reflective image-forming element is shown in Japanese Patent Application Laying Open (KOKAI) No. 1-220221. Further, a mirror face shape of the reflective image-forming element used instead of the above f.theta. lens is set as a proposal to an aspherical surface to preferably correct field curvature and linearity of an image.
In the optical scanner using such a reflective image-forming element, the light beam deflected by the deflecting means is returned by the reflective image-forming element onto a side of the deflecting means. Accordingly, it is necessary to dispose an optical path separating means for separating an optical path of the light beam from the deflecting means toward the reflective image-forming element from an optical path of the light beam from the reflective image-forming element toward the scanned face.
For example, it is considered as one of the optical path separating means that the light beam is incident to the reflective image-forming element in a direction inclined with respect to the rotational axis of a deflecting reflecting face in the deflecting means. In this case, an advancing direction of the deflected light beam is inclined with respect to a plane perpendicular to this rotational axis. In such a structure, the direction of the deflected light beam incident to the reflective image-forming element is not in conformity with that of the light beam reflected on the reflective image-forming element so that an optical path of the incident deflected light beam can be separated from an optical path of the reflected light beam. However, when the optical paths are separated from each other by only such a separating means, an angle of the light beam incident to the deflecting reflecting face with respect to the above plane must be increased to a certain extent to enable a layout of each of optical elements. When this incident angle is increased, the locus of a light spot on the scanned face is greatly curved so that a scanning line is greatly curved and the curved scanning line prevents a preferable optical scanning operation.
In various kinds of general known optical scanners relative to an optical printer, etc., a light beam deflected at an equal angular velocity is converged as a light spot on a scanned face to scan the scanned face. f.theta. lens is generally used as an optical system in which the deflected light beam is converged as a light spot on the scanned face and an optical scanning operation using this light spot is performed at an equal speed. Recently, an image-forming reflecting mirror having a linearity correcting function has been proposed and used instead of this f.theta. lens. For example, this image-forming reflecting mirror is shown in the above Japanese Patent Application Laying Open (KOKAI)No. 1-200221.
In an optical scanner having such an image-forming reflecting mirror, a Scanning line as a moving locus of the light spot to be straight tends to be curved. In the following description, this is called "a curve of the scanning line is caused".
In the optical scanner using the above image-forming reflecting mirror, the deflected light beam incident to the image-forming reflecting mirror and the deflected light beam reflected on the image-forming reflecting mirror are located on the same side with respect to the image-forming reflecting mirror. Accordingly, it is necessary to use an optical arrangement in which these light beams are separated from each other and the deflected-reflected light beam is not returned onto the side of a light source, but is guided to a scanned face side. There is a case in which the scanning line curve is necessarily caused in accordance with a certain optical arrangement.
Such a curve of the scanning line caused by the optical arrangement can be corrected by shifting a position of the image-forming reflecting mirror in a cross-scan corresponding direction to such an extent that no practical problems are caused. Otherwise, such a curve of the scanning line can be also corrected by inclining the image-forming reflecting mirror to such an extent that no practical problems are caused. However, each of optical elements must be arranged on an optical path from the light source to the scanned face with very high accuracy to correct the scanning line curve as designed. Accordingly, when errors in arrangement accuracy of each of the optical elements are accumulated, a very large curve of the scanning line is caused.
For example, such a curve of the scanning line causes a serious problem in a two-color printer, etc. for waiting information every color by a separate optical scanning operation. For example, when reference numeral L in FIG. 80 is set to an ideal scanning line, there is a case in which scanning lines L.sub.1 and L.sub.2 for writing information of each of two colors are curved in opposite directions. In such a case, a shift (d.sub.1 +d.sub.2) is caused at its maximum between the written color information in both end portions of an optical scanning region. Accordingly, a so-called color shift is clearly caused.